The invention relates to a device and method for electroplating metal workpieces. In particular, it relates to workpieces having axial symmetry, such as a circular workpiece.
In the practice of electroplating metal surfaces it is well-known that application of a modulating current has two major advantages: it improves the properties of the deposit, and it allows faster plating rates. Most common plating techniques that make use of a modulating current are those referred to as periodically reversed current, pulsed current, and asymmetric alternating current, which combines alternating current (AC) superimposed on direct current (DC). Studies of these techniques are described in the electroplating literature. For example, the use of sinusoidal AC current superimposed on DC current is described by C. C. Wan, H. Y. Cheh, and H. B. Linford in "The Application of Pulse Plating Techniques to Metal Deposition", Plating 61 (1974) 559, and by R. B. Snyder and H. Y. Cheh, Plating Surface Finish, 62 (1975) 786. Use of periodically reversed current is described by H. R. Despic and K. R. Popov in "Transport-Controlled Deposition and Dissolution of Metals", Modern Aspects Electrochemistry, 7 (1972) 199. Studies on pulse current are described by G. Perger and P. M. Robinson in Metal Finishing, 77 (12) (1979) 17.
In the electroplating techniques described above, it is critical to be able to periodically reverse the deposition process. The reversing step virtually halts dendritic growth of previously formed nuclei in the metal layer being deposited on the cathode surface, so that it has a beneficial effect in achieving a metal coating that is uniform and coherent. The technique that combines application of AC current superimposed on DC current is particularly suitable for obtaining high quality metal deposits. But this technique has certain disadvantages. For example, it requires a means for continuously stirring the plating solution and a signal generator for providing the alternating current to be superimposed on the DC current.
The present invention overcomes the problems described above. In this invention the cathode (the workpiece being plated) is rotated inside an anode structure submerged in the plating solution. As the cathode rotates, part of it continuously moves through a region of intense current fluxes created by passing DC current between the anode and cathode. The rotating cathode stirs the plating solution, which speeds up the plating rate. At the same time, with only part of the cathode continuously passing through the intense current flux region, the deposition process is periodically slowed down and intensified.